The Human Factor (Everyman's Library Classics)

From first page Avon paperback February 1979:

DOUBLE AGENT

In the low-key but treacherous domain of "the firm," Maurice Castle performs routine intelligence duties that are less than world-shattering. Then a new security chief, "a broom," is appointed. As he sweeps through the department, he uncovers a leak, a systematic betrayal involving the major powers in South Africa.

Castle's associate, Davis, had access to the files in question and his drinking, gambling, and incurable romanticism make him a likely double agent. Castle himself is above suspicion; he had always been very careful.

Except when he fell in love.